Power transmitting mechanism



Jan. 5 1943. T Y .1. UNDERHILL 2,307,475

POWER TRANSMITTING MECHANI SM Filed Nov. 29, 1941 WW I Patented Jan. 5, 1943 1 mews POWER TRANSMITTING MECHANISM Joseph L. Underhill, Indianapolis, Ind, assignor to Radio Corporationof America, a corporation of Delaware Application November 29, .1941, SerialNo. 420,953

12 Claims.

This invention relates to power transmitting mechanism, and more particularly to a driving mechanism for the turntable of phonograph apparatus.

It is well known that turntables of phonographic apparatus are subject to variations in load over a substantial range. Thus, for example, in a machine adapted for both recording sound on a blank record and reproduction of sound from a finished record, the load upon the turntable when a record is being cut in the blank is greater than the load upon the turntable when a reproducing needle is tracking the groove of the finished record. This difierence in load upon the turntable results in a diiferenc'e in speed thereof which causes a variation in pitch between the sound as recorded and the same sound as reproduced.

The primary object of my present invention is to provide an improved turntable drive which will not be subject to the aforementioned disadvantage.

More particularly, it is an object of my invention to provide an improved turntable drive mechanism the speed of which will not vary appreciably over a wide variation of loads upon the turntable.

Another object of my invention is to provide an improved turntable driving mechanism as aforesaid which will insure a constant load on the driving motor.

It is also an object of my invention to provide an improved turntable driving mechanism as above set forth which is simple in construction, inexpensive in cost, easy to assemble, and highly efficient in use.

In accordance with my invention, I provide a motor having a driving pulley on its shaft and a rubber tired idler pulley interposed between the motor driving pulley and the rim of the turntable. Thus, the motor pulley drives the idler which, in turn, drives the turntable. The idler carries a brake drum and is fioatingly mounted relative to a stationary brake member, a tension spring acting on the supporting member for the idler in such a manner as to pull the brake drum into pressing engagement with the brake shoe. When there is no load on the turntable, or when the load on the turntable is very light, the full tension of the spring acts on the idler to press the brake drum hard against the brake, but when the load on the turntable is increased, the turntable reacts on the idler in such a way that it reduces the effect of the tension spring and the pressure between the brake shoe and the brake drum is the motor at the point of contact between its' --drive pulley and the idler pulley over a desired variation of loads on the turntable. Alternatively, .it ispossible for the load at the point of contact between the drive pulley and the idler pulley to either increase or decrease with an increase of turntable load. By arranging for the load at the aforementioned point of contact to decrease slightly as the turntable load increases, it is possible to compensate for the increased slippage at the point of contact between the'idler and. the turntablerim when the turntable load is increased.

The novel features that I consider characteristic of my invention are set forth with particularity .in the appended claims. The invention itself, however, both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description, when read in con.- nection withth'e accompanying drawing, in .which Figure. lis a fragmentaryplan view of a phonograph employing a drive mechanism according 'to one formof my, invention,

Figure 2 is a side elevation thereof, and

Figures 3, 4 and 5 are diagrammatic views I illustrating the principle involved in the operation of a power transmission mechanism in accordance with this invention.

Referring more particularly to the drawing, wherein similar reference characters designate corresponding parts throughout, there is shown a motor board I carrying a bushing 3 in which is rotatably received the spindle of a turntable. 5 having a peripheral flange or rim 7. The motor board I is also provided with an opening 9 throughwhich extends the drivesh'aft l I of a suitable motor l3 fixed to the underside of the motor board I. The motor shaft H is provided with a driving pulley I5 from which torque is transferred to the turntable 5 by a rubber-tired, preferably metal, idler pulley ll interposed between and having engagement with the driving pulley l5 and the rim 1 of the turntable.

The idler pulley I7 is rotatably carried on the end of a supporting arm [9 which is pivotally mounted on a stud 2| carried by the motor board I. Also fixed to the motor board I is a bracket or the like 23 to which is rigidly secured a brake member, such as a brake shoe 25, adapted to cooperate with a brake drum 21 which may be either formed integral with or secured to the idler pulley IT. The brake drum 2'! is preferably of smaller diameter than the rubber tired idler ll. It will be noted that, by reason-of the pivotal mounting of the supporting arm l9, the idler I1 and its brake drum 2'! are floatingly supported relative to the brake shoe 25. A tension spring 29 having one end fixed to the motor board I and the other end secured to the supporting arm [9 constantly pulls the arm IS in a clockwise direction, as shown, to maintain the brake drum I! in pressing engagement with the brake shoe 25.

When there is no load, or very little load, on the turntable, the spring 29 pulls the brake drum 2'! against the brake member 25 with maximum pressure and produces a certain load on the motor I3. As the load on the turntable is increased, the increased load causes the turntable to apply a reactive force on the idler tending to cause the idler to ride around on the rim of the turntable in a' direction away from the brake shoe 25. Thus, the supporting arm l9 tends to move slightly away from the brake shoe 25 against the tension of the spring 29, thereby reducing the pressure of the brake shoe on the brake drum and cancelling out the increased load on the motor due to increase in turntable load. By proper brake design (that is, by suitably shaping the brake shoe 25 and selecting materials for the brake shoe 25 and the drum 2! to have a suitable coefficient of friction therebetween), and by properly selecting the tension of the spring 29, it is possible to maintain practically constant motor load between desired limits of turntable load. If slippage occurs at the point -of contact X between the turntable rim 1 and the idler pulley I! under turntable load, it could be compensated for by extra brake action causing extra slippage at the point of contact Y between the drive pulley l5 and the idler pulley II when the turntable load is light.

For a better understanding of how the pressure on the friction idler brake drum 29 is decreased by an increase in load on the turntable, and vice versa, reference is made to Figs. 3, 4 and 5. Let it be assumed that the motor drive pulley l5 rotates in a clockwise direction and, in turn, rotates the idler pulley I7 and the brake drum 2! in a counterclockwise direction, thereby causing the turntable to rotate in a clockwise direction similar to itself. As explained heretofore, if there is no. load on the turntable, the only force reacting on the motor pulley I5 is that due to the friction of the brake. However, when a load is applied to the turntable, an additional reaction force is applied to the idler pulley which tends to move the idler pulley to the right, as seen in Fig. l. A diagram of all of the forces is shown in Figs. 3, 4 and 5.

Let it beassumed, in the first case represented by Fig. 3, that the idler I1 is free to fioat'in a vertical direction and that the brake shoe 25 engages a brake drum 2! of the same diameter as the idler pulley IT. The force F1 represents the motor force tending to raise the idler upwardly; F5 represents the force due to the spring 29 tending to pull the idler downwardly against ,the brake 25; Fb is the reactionary force of a brake 25 against the idler brake drum 21; F1: is the resultant forceof the brake upon the idler in a tangential direction; and F2 is the reactionary force upon the idler ll due to the load on the turntable. It is the force F2 which varies with the turntable load and reduces the braking force Fb to keep the force F1 constant. This may be seen from the following analysis:

where K==the coefi'icient of friction between the brake shoe 25 and the brake drum 21. For a condition of balance or equilibrium,

Fs=Fb+F1+F2 (3) From Equation 3,

Fl=FsFbF2 (4) Now, substituting in Equation 5 the value of F1 derived from Equation 1,

Assuming that K=l, and substituting in Equation 9, it is found that Similarly, if it is assumed that K=0.5, it is found that V V Figure 4 illustrates an arrangement like that shown in Fig. 1, namely, with the brake drum 21 having a smaller radius ,A than the radius B of the idler H. The forces illustrated are the same as those in Fig. 3, the additional force F3 being that resulting in a tangential direction on the idler pulley I! as' a result of the tangential force F: resulting on the brake drum 21 in a tangential direction from the braking force Fb. In this alternative arrangement,

Substituting Equation 11 in Equation 10,

F, =F +%F; (12) Substituting Equation 2 in Equation 12 5 A F1=F2+ t) Now, substituting in Equation 13 the value of Fb derived from Equation 3,

A 1= z+- l 1 2) Whence,

' A A A 15 F =F KF, KF ,KF (15) A A A (1+ K)Fi=- KF,+(1 )F2 16) B F, 1) F K F1= (1s) Assuming that A AK it follows from Equation 18 that Similarly, where AK .40, then Also, where l F E AK 2 1 3 3 In the foregoing illustrations, it is assumed that the brake shoe 25 is applied to the brake drum along a radius of the brake drumwhich is substantially 90, or perpendicular, to a line L-L joining the center of the turntable and the center of "the idler H, as shown in Figure 1. However, the brake shoe may be applied tothe brake drum at any other suitable point, for example, l5 around the brake drum from the line L-L, as shown in Fig. 5. If the angle 11 represents the angle between the radius along which the force Fb acts on the brake drum and the line L-L, then the force Fe shown in Fig. 5 is the component of the force Ft acting ,in the direction of the forces F1 and F2, and

By substituting in Equation 13, which applies equally well to Fig. 5, the value of Fb derived from Equation 19,.it1fo11ows that Now, substitutingthe value of Fe derived from Equation 20 in Equation 21,

F1=F2+( F,F1a (22) AK AK AK F1=F2 B cos a) s B cos a) 1- B cos a Solving for F1, it is found that AK AK B cos a) B cos 0;) F1= *AK (24) 1 B cos 0:

Assuming that AK B we a then F 8 F1 If AK 1 B cos a 2 then On the other hand, where A JZi B cos 04 4 1 2 From the foregoing analyses, it will be apparent that by a suitable relation between the coefficient of friction between the brake shoe 25 and the brake drum 2! and the radii of the brake drum 2! and the idler pulley H, the load on the motor I 3 can be held constant with the turntable load or can be made to increase or decrease in proportion to, or as a function of, the turntable load. In one arrangement which was constructed and tested, the idler l? was 2% inches in diameter and had a brake drum 21' of cylindrical form and a diameter of approximately 2 inches. The brake shoe 25 was made of poplar and was arranged to press against the drum as illustrated in Fig. 5, or about 45 around the drum from the line L-L. In this construction, there was a reduction of speed of 0.455 percent to 0.3 percent when the turntable load was increased from 0 to 56 inch ounces. This may be contrasted with a change of 2.58 percent upon a change of turntable load from 0 to 60 inch ounces when the brake member 25 was not used. In another form of the invention, a chrome leather brake was used and arranged as in Fig. 5. This arrangement showed a change of speed of from 0.6 percent to 0.3 percent over a turntable load range of from 0 to 60 inch ounces. Using a brake drum with a 60 V groove and a chrome leather brake fitting the V groove of the drum and set at approximately to the line L-L, as shown in Fig. 1 and as illustrated by Fig. i, there was a loss of speed of only 0.5 percent as the turntable load increased from 0 to 56 inch ounces.

The invention heretofore described may not only be employed to compensate for added load on the turntable to thereby keep the load on the motor constant, but it is also possible to obtain over-compensation. Thus, by using a 30 v groove brake drum and a poplar brake set at 90 ,to-the line L-L,the speed of the turntable -actually increased about 1 percent when the load-of the turntable was increased from to 60 inch ounces.

From the foregoing description it will be apparent to those skilled in the art that I have provided a novel power transmitting mechanism by means of which the load on the motor may be maintained constant with variations in load on the driven member. Although I have shownand described several arrangements embodying one form of my invention, it will be apparent to those skilled in the art that many other variations are possible. I therefore do not wish to be limited except insofar as is made necessary by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. In combination, a driving member, a driven member subject to variations in load, an idler interposed between said members for transferring torque therebetween, braking means associated with said idler for applying a restraining force thereto, and means mounting said idler for movement relative to said driven member and also relative to said braking means in response to variations in load on said driven member, said load variations causing said idler to move relative to said driven member and thereby to effect movement of said-idler relative to said braking means for varying said restraining force as a function of the load on said driven member.

2. In combination, a driving member, a driven member subject to variations in load, an idler interposed between said members for transferring torque therebetween, braking means associated with said idler for applying a restraining force thereto, and means floatingly supporting said idler relative to said braking means whereby said idler is adapted to vary its contact with said braking means in accordance with variations in load on said driven member to thereby Vary said restraining force as a function of said load.

3. In combination, a driving member, a driven member subject to variations in load, an idler in.- terposed between said members for transferring torque therebetween, said idler including a brake drum, a brake member, means floatingly supporting said idler relative to said brake member, and means constraining said idler into position such that said brake drum is in pressing engagement with said brake member whereby said brake memher is adapted to apply a restraining force to said idler, said brake drum being adapted by reason of the floating support of said idler to vary its engagement with said brake member in response to variations in load on said driven member to thereby correspondingly vary said restraining force as a function of said load.

4. In a phonograph, the combination of a turntable subject to variations in load, a motor including a driving pulley, a floating friction idler interposed between and having engagement with said driving pulley and the rim of said turntable, said idler including a brake drum, a brake member, and means constraining said idler into pressing engagement with said brake member whereby said brake member is adapted to apply a re straining force to said idler, said brake drum being adapted by reason of said floating idler to vary its engagement with said brake member in response to variations in load on said turntable to thereby correspondingly vary said restraining force as a function of said load.

5. In a phonograph, the combination of a including a driving pulley, a friction idler interposed between and having engagement with said driving pulley and the rim of said turntable, said idler including a brake drum, a brake member, imeans floatingly supporting said idler relative to said brake member, and means constraining said supporting means into position such that said brake drum is in pressing engagement with said brake member whereby said brake member *is adapted to apply a restraining force to said idler, said brake drum being adapted by reason of the floating support of said idler to vary its engagement with said brake member in response to variations in load on said turntable to thereby correspondingly vary said restraining force as a function of said load.

6. In a phonograph, the combination of a turntable subject to variations in load, a motor including a driving pulley, a friction idler interposed gbetween and having engagement with said pulley and the rim of said turntable, said idler including a brake drum, a brake member, a pivotally mounted supporting arm carrying said idler in floating relation to said brake member, and spring means ;,=connected to said arm and arranged to constrain said arm to a position such that said brake drum is in pressing engagement with said brake member whereby said brake member is adapted to apply a restraining force to said idler, said brake drum being adapted by reason of said floating idler to vary its engagement with said brake member in response to variations in load on said turntable to thereby correspondingly vary said restraining force as a function of said turntable load.

'I '7. The invention set forth in claim 6 characterized in that said brake member is so located with respect to said brake drum that it engages said brake drum at a point on the circumference thereof from which a radius thereof is at an angle 40' to a line joining the centers of said turntable and said brake drum.

8. The invention set forth in claim 6 characterized in that said brake member is so located with respect to said brake drum that it engages said brake drum at a point on the circumference thereof from which a radius thereof is at an angle of substantially 45 to a line joining the centers of said turntable and said brake drum.

. 9. The invention set forth in claim 6 characterized in that said brake member is so located with respect to said brake drum that it engages said brake drum at a point on the circumference thereof from which a radius thereof is at an angle of 5 substantially 90 to a line joining the centers of said turntable and said brake drum.

10. The invention set forth in claim 6 characterized in that said idler and said brake drum are concentric and characterized further in that said idler has a different diameter than said drum.

11. The invention set forth in claim 6 characterized in that said idler and said brake drum are concentric, characterized further in that said idler has a different radius than said drum, and

characterized still further in that said brake member is so located with respect to said brake drum that it engages said brake drum at a point on the circumference of said drum from which a radius thereof is at an angle to a line joining the centers of said turntable and said drum.

12. The invention set forth in claim 6 characterized in that said idler and said brake drum are concentric, characterized further in that said turntable subject to variations in load, a motor idler has a different radius than said drum, characterized further in that said brake member is so located with respect to said brake drum that it engages said brake drum at a point on the circumference of said drum from which a radius thereof is at an angle to a line joining the centers of said turntable and said drum, and characterized still further in that the force applied by said driving pulley to said idler is given by the equation K 3 cos a where F1=f0rce applied by said driving pulley to said idler,

member,

A =radius of said brake drum, B =radius of said idler, and

a: =said angle.

JOSEPH L. UNDERHILL. 

